Adelophis foxi and its congener, A. copei, share several morphological characteristics not seen in Thamnophis, including the presence of only five supralabial scales (vs six or more in all Thamnophis) and a lack of reduction in dorsal scale row numbers posteriorly. In addition, both species of Adelophis have striping patterns unlike those of any Thamnophis, although they also differ from each other in this respect (Rossman and Blaney, 1968). However, all of these traits can be interpreted as autapomorphies of Adelophis as a whole or of A. foxi and A. copei individually. If this is the case, these traits would have no bearing on the relationships of Adelophis to other taxa. Interestingly, both species of Adelophis possess the one obvious morphological synapomorphy for Thamnophis, an undivided anal plate (Rossman and Blaney, 1968). Interestingly, Adelophis foxi is nested within Thamnophis by DNA analysis (De QUEIROZ et al. 2002).

References

Heimes, P. 2016. Snakes of Mexico. Chimaira, Frankfurt, 572 pp

Queiroz, Alan de; Robin Lawson and Julio A. Lemos-Espinal 2002. Phylogenetic Relationships of North American Garter Snakes (Thamnophis) Based on Four Mitochondrial Genes: How Much DNA Sequence Is Enough?. Molecular Phylogenetics and Evolution 22: 315-329 - get paper here

Rossman, Douglas A. & Blaney, R. M. 1968. A new Natricine snake of the genus Adelophis from western Mexico. Occasional papers of the Museum of Zoology, Louisiana State University (35): 1-12 - get paper here